Composite resin restoration of posterior teeth necessitates a reassessment of cavity insulation and dentine conditioning. The primary function of cavity insulation under composite resin restorations is to prevent the formation of a bacterial, fluid-filled gap nearest the cavity walls. Gap formation can contribute not only to sensitivity to cold but also to pulpal complications and secondary caries. Unfortunately, none of the currently available systems for cavity insulation fulfil this basic requirement. The evidence suggests, however, that the most effective insulator is a very thin liner which does not pool in retention grooves and which can cover the entire cervical wall as well as the remaining cavity walls, except for etched enamel. Such a liner should not bond to composite resin and thus be detached from the dentine during polymerization contraction of the composite. Nor should it be leached away by the oral fluids. In order to preserve the liner, the air-filled contraction gap which forms between liner and composite should be impregnated with resin immediately after the major setting contraction of the composite. In this context the use of retention grooves in Class II and Class V cavities is recommended, to minimize the effect of thermal and occlusal stress. Combined with etched enamel walls, well-placed retention grooves can reduce the risk of gap formation and creep due to mechanical stress and the deformation potential of the tooth structure and the filling. The smear layer can disappear entirely and be replaced by fluid and bacteria. It should therefore be removed, but the smear plugs blocking the apertures of the dentinal tubules should be left undisturbed and reinforced by impregnation with fluoride. Some hygroscopic expansion may be an advantage in a composite. It is also to be hoped that the natural defence mechanisms of the body will block gaps and tubular apertures and protect the pulp from shortcomings in clinical techniques.